1. Field of the Invention
This invention relates to an improved process for the polymerization of olefins to provide linear polymers and copolymers, said process employing an improved catalyst comprising tetra(neophyl) zirconium, or its reaction products with metal oxides, for use therein. More specifically, this invention relates to a process for the homopolymerization and copolymerization of ethylene, propylene, butene-1 and higher 1-olefins in which the catalyst is either tetra(neophyl) zirconium or, preferably, the product obtained by the reaction of tetra(neophyl) zirconium with a partially hydrated metal oxide surface. The metal oxide can be an oxide selected from a metal of Groups II(a), III(a), IV(a), or IV(b) of the Bohr Periodic Table of the Elements such as fumed Al.sub.2 O.sub.3, fumed TiO.sub.2, fumed SiO.sub.2, MgO, coprecipitated Al.sub.2 O.sub.3.ZrO.sub.2 or SiO.sub.2.Al.sub.2 O.sub.3 in each case free from merely absorbed H.sub.2 O but partially surface-hydrated. The most active, preferred catalyst is neophyl zirconium aluminate on alumina prepared from the reaction of tetra(neophyl) zirconium with fumed Al.sub.2 O.sub.3 having 0.5 to 1.5% water of hydration on its surface prior to reaction with tetra(neophyl) zirconium.
2. Prior Art
In 1954 and 1955 pioneering advances in olefin polymerization catalysts were disclosed by Karl Ziegler and associates at the Max-Planck Institute for Coal Research in Mulheim, Germany, and by Arthur Anderson and associates in the laboratories of E. I. du Pont de Nemours and Company in Wilmington, Delaware. These new catalyst systems, now frequently termed coordination catalysts, were based on transition metal salts (e.g. titanium, zirconium or vanadium halides) which had been converted into reduced valence states by reaction with a variety of alkylating or arylating substances, usually simple organometallic compounds of a metal of Groups I, II or III of the Periodic Table of Elements. It is believed that the mechanism of catalyst production involves alkylation (or arylation) of the transition metal halide followed by rapid decomposition of the unstable, transitory transition metal-alkyl (or aryl) compounds to give more stable complex products of lower valency which actively coordinate with and polymerize olefins by a coordination-polymerization mechanism. Unlike the commercial polyethylene or any polypropylene previously known prepared by free-radical or ionic catalyst, polyolefins prepared with coordination catalysts are of very high molecular weight and linear and highly ordered, thus exhibiting, in the case of homopolymers, such a high degree of chemical structural regularity and linearity that they are highly crystalline and exhibit high crystalline melting points, making them extremely valuable as textile fibers, films, and molded articles of commerce. Generally, however, it has been necessary in the case of these coordination catalysts to devise processes to remove the catalyst residues, which comprised the transition metal halides, since the residues were present at such levels as to discolor the polymer and the halide was too corrosive in subsequent fabrication machinery.
More recently some more stable organometallic transition metal complexes, usually including a halide anionic ligand or a neutral Lewis Base ligand, have been disclosed in patents of Gunther Wilke of the Max-Planck Institute and by patents of several researchers in the laboratories of I.C.I. in England
Illustrative of the Wilke patents are U.S. Pat. Nos. 3,379,706, 3,424,777, 3,432,530, 3,540,728, 3,468,921 and 3,536,740. In all of these Wilke patents the hydrocarbyl groups attached to a transition metal are members of the class of .pi.-allylic compounds characterized by the structure ##STR1## in which the R-radicals may be H-- or any alkyl, aryl or alkaryl radicals or R.sub.1 and R.sub.4 may together form a ring comprising methylene groups. Interactions between the .pi. electrons and the electrons of the transition metal presumably occur affecting the stability and chemical reactivity of the complexes. These metal-.pi.-allyl compounds are reacted with Lewis acids, such as HX, where X is halide, or Lewis Bases, such as tertiary amines or phosphines, to form complexes showing activity as olefin oligomerization or polymerization catalysts. However, polymerization reactions using such catalysts generally must be conducted as slurry polymerizations at low temperatures, because of marginal thermal stability and low solubility of the .pi. allyl complexes, and have been found less than fully satisfactory in the yields and molecular weight of polymers produced. Furthermore, because of their corrosive and often toxic nature, the catalyst residues must be removed from the polymeric products by time-consuming and costly procedures in order to provide products of general utility and safety in commerce.
One improvement on the use of these .pi.-allylic transition metal complex catalysts in olefin polymerization is disclosed in U.S. Pat. No. 3,732,198 of Whitely et al., assignors to I.C.I., who disclose the polymerization of ethylene with a combination of a classical coordination catalyst with a transition metal complex of a .pi.-allylic compound.
The patents arising from the work of researchers at I.C.I. in England are illustrated by U.S. Pat. Nos. 3,681,317, 3,740,384, 3,738,944 and British Pat. No. 1,314,828. All of these involve tetra(benzyl)-transition metal compounds (e.g. tetra(benzyl) zirconium) complexed with anionic ligands (e.g. halide) and/or neutral ligands (e.g. pyridine) as ethylene polymerization catalysts. In certain cases there are disclosed as ethylene polymerization catalysts the reaction products of tetra(benzyl)-zirconium compounds with inorganic oxides free of absorbed water but containing surface HO- groups. Reasonable thermal stability is achieved with these substances. They apparently yield high molecular weight polyethylene but the polymerization rate and efficiency and polymer yield obtained with those catalysts in processes for the polymerization of ethylene are generally not as good as with classical coordination catalysts, and hence again it would be necessary to remove catalyst residues in order to produce polyethylene suitable for general fabrication techniques.